An imaging device in the form of a printer or copier typically creates images using combinations of four colors of marking agents or colorants, such as cyan, magenta, yellow and black (CMYK). The images are created based on image data which assigns at least one of the four colors and a numerical color intensity or input color value to each picture element or pixel in the image.
A problem is that, due to manufacturing variations, different imaging devices can output different intensities of color based on identical image data. The density of the toner laid down on the print medium determines the color intensity. The denser or thicker the toner is laid down on a white print medium such as paper, the less white is visible through the toner on the paper. Consequently, the denser the toner, the less the lightness of the toner color, and the greater the intensity of the toner color.
Because there is such variation in toner density laid down by different imaging devices based on identical image data, color intensities that are output by some imaging devices can be outside of an acceptable range. Thus, in order to ensure that each imaging device outputs color intensities that closely correspond to the color intensities specified by the image data, each imaging device should be individually calibrated to output appropriate color intensities.
One approach in calibrating an imaging device is to use a tone reproduction curve (TRC), which is a modeled mathematical relationship between the input color values and the colorant amounts that must be sent to the imaging device in order to produce the intensities specified by the input color values. The TRC is a continuous curve on a plot of input color values versus output colorant values that represents a best fit of the discrete data points matching each input color value with an output colorant value that, when rendered on the given device, produces the intensity specified by the input color value.
A processor of the imaging device calculates a separate TRC for each of the colors or separations of the imaging device. The TRCs are used to calibrate the imaging device. More particularly, once such TRCs are established for an imaging device, the TRCs can be used to correlate input color values with imaging device output image colorant values or color intensities. In addition, a multi-dimensional look up table or LUT is often calculated to account for interactions among the colorants and to accommodate different input color spaces, for example, CIELAB or sRGB. Numerous techniques exist in the prior art for deriving LUTs and TRCs for printer calibration and characterization.
A processor of the imaging device calculates a separate TRC for each of the colors or separations of the imaging device. The TRCs are used to calibrate the imaging device. More particularly, once such TRCs are established for an imaging device, the TRCs can be used to correlate input color values with imaging device output image colorant values or color intensities. In addition, a multi-dimensional look up table or LUT is often calculated to account for interactions among the colorants and to accommodate different input color spaces, for example, CIELAB or sRGB. Numerous techniques exist in the prior art for deriving LUTs and TRCs for printer calibration and characterization.
Although the following discussion focuses on TRC construction as an example, much of the discussion also applies to LUT construction. In order to gather the data required to construct the TRCs, optical measuring devices are used to measure the color values of the images output by an imaging device.
In copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of print is to use the color-measuring device to measure an array of artificially created test patches. Each patch is intended to be of a respective predetermined desired density. Generally each patch is about an inch square or less and is printed as a uniform area. The actual density of the printing colorant (toner or ink) in the test patch is then optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
Each of the test patches is formed with a different combination of a colorant (C, M, Y or K) and a numeric input color value. The input color value specifies the desired colorant density, and consequently, the desired output lightness color value or color intensity. The density of the colorant on the test patches varies as a function of the input color value. Thus, optically measuring the density of the patches provides an indication of the input color values with which the patches were made. The denser the colorant on the test patch, the more light will be absorbed by the colorant, and the less light will be reflected back to the optical color-measuring device. This indicates a greater intensity provided by the colorant. The printed test patches are moved past the color-measuring device, and the light absorption of the test patch is measured.
The measurements of the test patches and the input color values used to construct them are used to calculate the TRC. The accuracy of the TRC increases with the number of data points that it is based upon. Measurement error drops by a factor of the square root of the number of measurements. However, from a cost viewpoint, it is desirable to minimize the number of test patches that are printed, since printing test patches consumes a significant amount of colorant.
One source of inaccuracies in TRCs is spatial variation of operation of the imaging device being calibrated. In particular, even a single imaging device does not always deposit a same amount of marking agent for a given input color value at different spatial locations on the page. Rather, the amount of marking agent that is deposited in a group of pixel locations for a given color value can depend upon where on the page the pixel locations are situated. For example, an imaging machine may print an entire horizontal or vertical streak or band of pixel locations with more or less marking agent than is called for by the corresponding input color values. Horizontal or vertical streaking can arise due to variations in raster output scanning spot size across the field, laser diode variations, LED bar power variation, or photoreceptor belt sensitivity variations, among other reasons. Other printing technologies (e.g., thermal inkjet and acoustical ink printing) also have streaking artifacts that fall within the scope of this discussion.
Although the streaks can be one-time occurrences that are not likely to reappear, the streaks can affect calibration operations. In particular, banding or streaking can artificially inflate or deflate the color output values used to plot the TRCs. As a result, subsequent calibration steps will result in inaccurate color reproduction.
FIG. 1 is a plot of input color values that are used to produce single-colorant test patches versus corresponding lightness (L* in CIELAB color space) output color values that are measured from the test patches. It can be assumed that the input color value is a grey scale value ranging from 0 (white) to 255 (black). In the example of FIG. 1, streaking has caused certain outlying data points 10, or outliers, which are measured from test patches that are within the same column of the array of test patches. These outliers 10 are clearly anomalous when compared to the other data points.
A TRC 12 is calculated to fit the data, including the outliers 10. The TRC 12 is a compromise between the outliers 10 and the remaining data, and therefore does not fit the remaining data particularly well. The poor fit is a problem because, while the outliers 10 are transitory, the remaining data is reproducible, and is therefore the best predictor of the future color output characteristics of the imaging machine. Nevertheless, the TRC 12 represents the best known fit of the available data (which includes the outliers), and is subsequently used to calibrate the imaging device. As a result, the actual color output of the imaging device is not as close to the intended color output as is desirable. For example, in the example of FIG. 1, if the imaging device desires to produce an output color value of 45, then it will use an input color value of 90 in accordance with the TRC 12. However, the input color value of 90 will in fact result in an output color value of approximately 37.
One prior art attempt to increase the accuracy of TRCs is to randomize the order of the input color values with which the test patches are printed. In such an approach, test patches in a same row or column are not printed with consecutive input color values (i.e., input color values that are close to each other), as is evident from the outliers 10 being spread out along the TRC 12 in FIG. 1. This prevents particular regions of the TRC from being unduly warped from the effects of the vertical or horizontal streaking. However, this does not eliminate the underlying problem caused by streaking, namely that the outlying output color values will cause inaccuracy of the TRC.
Another approach to reducing the inaccuracies due to outlying data consists of identifying and discarding the outlying data before constructing the TRC. However, in order to conserve marking agent, the input color values are sampled very sparsely over the range of input color values to be represented by the sampled input color values. That is, the sampled input color values are widely spread out and each sampled input color value is very different from its adjacent sampled input color values. This is particularly true when dealing with test patches formed of multiple color components, which causes the sampled input color values to be separated in multi-dimensional space. Thus, throwing out measured color values would leave unacceptable gaps in the data with which the TRC is formed. Moreover, the outlying data, although anomalous, still contains useful information.
What is needed in the art is a new and improved technique for process control, in particular, for establishing a tone reproduction curve or multidimensional LUT. In particular, what is needed is a technique for establishing a tone reproduction curve that is not affected by streaks in the printer output when test patches are printed. Other advantages of the present invention will become apparent from the following description, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.